Controllable two layer birefringent optical component

ABSTRACT

An optical component ( 181 ) comprises a first birefringent layer ( 203 ) connected to a second birefringent layer ( 170 ) by a curved interface ( 206 ). An optical axis ( 19 ) passes through the first and the second layer. The second birefringent layer ( 170 ) has molecules movable between a first orientation and a second orientation relative to the optical axis. The refractive index of the second birefringent layer ( 170 ) is dependent upon the orientation of the modules.

The present invention relates to a birefringent optical component, amethod of manufacturing such a component, and devices including suchcomponents. The component is particularly suitable for, but not limitedto, use as a variable focus lens in optical scanning devices.

Optical pickup units for use in optical scanning devices are known. Theoptical pickup units are mounted on a movable support for scanningacross the tracks of the optical disk. The size and complexity of theoptical pickup unit is preferably reduced as much as practicable, inorder to reduce the manufacturing cost and to allow additional space forother components being mounted in the scanning device.

Modern optical pickup units are generally compatible with at least twodifferent formats of optical disk, such as the Compact Disc (CD) and theDigital Versatile Disc (DVD) format. Recently proposed has been theBlu-ray Disk (BD) format, offering a data storage capacity of around 25GB (compared with a 650 MB capacity of a CD, and a 4.7 GB capacity of aDVD).

Larger capacity storage is enabled by using small scanning wavelengthsand large numerical apertures (NA), to provide small focal spots, (thesize of the focal spot is approximately λ/(NA), so as to allow thereadout of smaller sized marks in the information layer of the disk. Forinstance, a typical CD format utilizes a wavelength of 785 nm and anobjective lens with a numerical aperture of 0.45, a DVD uses awavelength of 650 nm and a numerical aperture of 0.65, and a BD systemuses a wavelength of 405 nm and a numerical aperture of 0.85.

Typically, the refractive index of materials vary as a function ofwavelength. Consequently, a lens will provide different focal points anddifferent performance for different incident wavelengths. Further, thediscs may have different thickness transparent layers, thus requiring adifferent focal point for different types of discs.

In some instances, storage capacity is further increased by increasingthe number of information layers per disc. For example, a dual layerBD-disc has two information layers separated by a 25 μm thick spacerlayer. Thus, the light from the optical pickup unit has to travelthrough the spacer layer when focusing on the second information layer.This introduces about 255 mλ rms (0.255λ root mean square) of sphericalaberration, the phenomenon that rays close to the axis of the convergingcone of light have a different focal point compared to the rays on theoutside of the cone. This results in a blurring of the focal spot, and asubsequent loss of fidelity in the read-out of the disc.

To enable dual layer readout and backward compatibility (i.e. the sameoptical system being used for different disc formats), polarizationsensitive lenses (PS-Lenses) have been proposed to compensate forspherical aberration. Such lenses can be formed of a birefringentmaterial, such as a liquid crystal. Birefringence denotes the presenceof different refractive indices for the two polarization components of abeam of light. Birefringent materials have an extraordinary refractiveindex (n_(e)) and an ordinary refractive index (n_(o)), with thedifference between the refractive indices being Δn=n_(e)−n_(o). PSlenses can be used to provide different focal points for a single ordifferent wavelength(s) by ensuring that the same or differentwavelengths are incident upon the lens with different polarization.

A new trend in optical storage is multi-layer or 3D recording. Oneexample of this technique is based upon stacking of multiple fluorescentlayers, thus increasing the data storage capacity on one disc.Multi-layer stacks also require light paths that enable accuratefocusing of the laser beam at a plurality of specific depths within adisc. Although actuators, present in the optical disc scanning system,do enable the objective lens to move within a certain distance rangefrom the disc (and so enable the focal point to move over a range ofdistances), such a movement range is limited, and it can not provide thefocal depth range required for all proposed multi-layer recordingsystems.

It is an aim of embodiments of the present invention to provide animproved optical component which addresses one or more of the problemsof the prior art, whether referred to herein or otherwise.

It is an aim of particular embodiments of the present invention toprovide an optical component comprising two birefringent materials, theoptical function of the component being adjustable, as well as a methodof manufacturing such a component. Particular embodiments provide anoptical lens with a focal point that can be controllably varied over apredetermined range of depths

In a first aspect, the present invention provides an optical component(181) comprising a first birefringent layer (203) connected to a secondbirefringent layer (170) by a shaped interface (206), an optical axis(19) passing through the first and the second layer, at least the secondbirefringent layer (170) having molecules movable between a firstorientation and a second orientation relative to the optical axis, therefractive index of the second birefringent layer being dependent uponthe orientation of the molecules.

By providing an optical component having two such materials, the opticalfunction defined by the interface can be changed by changing theorientation of the molecules. For instance, if the shaped interface iscurved, the lens capability provided by the interface can be altered bychanging the orientation of the molecules.

In another aspect, the present invention provides an optical scanningdevice (1) for scanning an information layer (4) of an optical recordcarrier (2), the device (1) comprising a radiation source (11) forgenerating a radiation beam (12, 15, 20) and an objective system (18)for converging the radiation beam on the information layer, wherein thedevice (1) comprises an optical component (181), the optical componentcomprising a first birefringent layer (203) connected to a secondbirefringent layer (170) by a shaped interface (206), an optical axis(19) passing through the first and the second layer, at least the secondbirefringent layer (170) having molecules movable between a firstorientation and a second orientation relative to the optical axis, therefractive index of the second birefringent layer being dependent uponthe orientation of the modules.

In a further aspect, the present invention provides a method ofmanufacturing an optical component (181) comprising a first birefringentlayer (203) and a second birefringent layer (170), the methodcomprising: providing a first birefringent layer with a shaped surface(206); providing a second birefringent layer (170) adjacent to theshaped surface (206) of the first birefringent layer;

-   -   wherein the molecules of the second birefringent layer are        arranged to be movable between a first orientation and a second        orientation relative to an optical axis (19) passing through the        first birefringent layer (203) and the second birefringent layer        (170).

In another aspect, the present invention provides a method ofmanufacturing an optical scanning device (1) for scanning an informationlayer (4) of an optical record carrier (2), the method comprising:providing a radiation source (11) for generating a radiation beam (12,15, 20);

-   -   providing an objective system (18) for converging the radiation        beam on the information layer; and providing an optical        component (181), the optical component comprising a first        birefringent layer (203) connected to a second birefringent        layer (170) by a shaped interface (206), an optical axis (19)        passing through the first and the second layer, at least the        second birefringent layer (170) having molecules movable between        a first orientation and a second orientation relative to the        optical axis, the refractive index of the second birefringent        layer being dependent upon the orientation of the modules.

For a better understanding of the invention, and to show how embodimentsof the same may be carried into effect, reference will now be made, byway of example, to the accompanying diagrammatic drawings in which:

FIG. 1 illustrates a cross sectional view of an optical component inaccordance with a preferred embodiment of the present invention;

FIGS. 2A-2F illustrate method steps in the formation of a first portionof a liquid crystal lens in accordance with a preferred embodiment ofthe present invention;

FIGS. 3A-3D illustrate method steps in the formation of the finalportion of a liquid crystal lens in accordance with a preferredembodiment of the present invention;

FIG. 4 illustrates a device for scanning an optical record carrierincluding a liquid crystal lens in accordance with an embodiment of thepresent invention;

FIG. 5 illustrates how the optical system of the scanning device shownin FIG. 4 may be used with different polarization of light to scandifferent layers within a dual layer optical record carrier; and

FIGS. 6A and 6B show cross sectional views of the liquid crystal lensillustrated in FIG. 1, with different orientations of the second layerof liquid crystal.

Optical components (or portions of optical components, optical elements)can include curved surfaces so as to focus light (e.g. a convex lens) ordisperse light (e.g. a concave lens). Birefringent optical componentswith curved surfaces will provide different focussing or dispersiveeffects, dependent upon the angle at which the polarized radiation beamis incident on the optical component.

Equally, optical functions of other components are provided by othershaped (i.e. non-planar) surfaces such as step functions and gratings.

The present inventors have realized that, by providing an additionalbirefringent material adjacent to the curved (or otherwise shaped)surface, with the orientation of the birefringence of the additionalmaterial being able to be altered in a controlled manner, then it ispossible to controllably alter the optical functions (e.g. the power ofthe lens formed by the curved surface) of the optical component.Further, as birefringent materials provide different refractive indicesto different polarisations of light, then different functions may alsobe realized by providing different polarisations of light incident uponthe optical component. Thus, such an optical component can providedifferent optional functionality by both changing the angle of incidenceof the polarized radiation, and also by altering the orientation of atleast one of the birefringent layers.

Consequently, for differently shaped surfaces, such as step structuresand gratings, the optical function of the components can be controllablyaltered by changing the orientation of the additional birefringentmaterial. Further, as both materials are birefringent different opticalfunctions can also be realized by providing light at different incidentpolarisations.

In inorganic birefringent materials (e.g. a crystal such as calcite) theatomic structure is non-symmetric. This leads to an anisotropy in thephysical constants of materials in different directions. One of those isthe refractive index. Consider a polarized beam of light traversingalong different optical axis. There will be one optical axis in which adifferent refractive index will be observed upon traversingperpendicularly and parallel to the optical axis. In general, but notalways, two out of three axes have a refractive index that is higherthan the refractive index of the third axis.

In organic crystals, such as a liquid crystal, a similar phenomenonoccurs although one cannot talk about a difference in the atomicstructure. Generally, although not always, two out of three axes have arefractive index that is lower than in the third axis.

The direction in which the molecules of a liquid crystal are aligned iscalled the director. Light propagating with its plane of polarizationparallel to the director experiences the extraordinary refractive index,n_(e).

FIG. 1 illustrates an optical component 181 in accordance with apreferred embodiment of the present invention. The optical component 181comprises a first layer of birefringent material 203 shaped as a lens.In this particular embodiment, the birefringent material 203 is shapedas a planoconvex lens, the convex portion of the lens being defined by acurved surface 206. The lens is formed as a solid body e.g. apolymerized liquid crystal.

The planar side of the lens is connected to a transparent electrode 150.The electrode is formed of a glass substrate covered with a layer of thetransparent conductor ITO (Indium Tin Oxide).

A layer 170 of a second birefringent material separates the firstbirefringent material 203 from a second transparent electrode 160. Thesecond electrode is again formed from glass and ITO. The secondbirefringent material 170 is arranged such that the orientation of thebirefringent properties of the material can be controllably altered.

In this particular example, both of the birefringent materials areformed from liquid crystal. The ridged body of the first birefringentmaterial is formed of a polymerized liquid crystal. The secondbirefringent material is a liquid crystal in the nematic phase. Themolecules of the second birefringent material are arranged to be movablebetween two different orientations.

The first orientation of the nematic liquid crystal is determined by oneor more alignment layers placed on at least one of the surfacessurrounding the nematic liquid crystal. The alignment layers here areformed of polyimide (PI). In this particular embodiment, two alignmentlayers are utilized. Each alignment layer is on an opposite surface ofthe enclosure surrounding the liquid crystal. Each of these surfacesextends substantially perpendicularly to the optical axis 19 (at leastin the immediate vicinity of the optical axis 19). In particular, afirst alignment layer 162 is located on the internal surface of theelectrode 160. The other alignment layer is placed on the surfaceopposite to the electrode i.e. upon the curved surface 206.

These alignment layers can take any preferred orientation with respectto each other e.g. they could be parallel, or indeed at anypredetermined angle with respect to each other. The directors within theliquid crystal tend to align with the orientation of the alignmentlayer. This then defines the first orientation of the directors i.e. ofthe molecules within the liquid crystal (and hence defines theorientation of the birefringent properties of the material 170).Further, these alignment layers can be orientated at any predeterminedangle with respect to the orientation of the birefringent materialwithin the first layer 203.

In this particular embodiment, the first material is aligned withdirectors substantially perpendicular to the optical axis. The alignmentlayer on the first material is also orientated so as to be perpendicularto the optical axis. Further, it is oriented so as to be simultaneouslyperpendicular to the orientation of the bireringent material in thefirst layer 203. In contrast, the alignment layer 162 upon the electrodeis arranged so as to be parallel to the alignment of the first material(and again also perpendicular to the optical axis 19).

In consequence, as the two alignment layers are oriented at 90° to eachother, the liquid crystal in the nematic phase forming the second layer170 will be arranged in the twisted nematic state. In other words, thedirectors of the liquid crystal rotate with distance along the opticalaxis. The directors of the liquid crystal in the second layer adjacentto the alignment layer 162 will be parallel to the directors in thefirst layer 203. However as a function of distance along the opticalaxis 19, the orientation of the directors in the second layer 170gradually changes, with the directors gradually rotating until, at thecurved interface 206, the directors are perpendicular to the directorsof the first layer 203.

This 90° rotation of directors within the second layer means that thebirefringence of the portion of the layer adjacent to the electrode 160will be different to that portion adjacent to the curved interface 206.In particular, the birefringent properties will be rotated through thesame 90° experienced by the directors. Further, polarized radiationpassing through the optical component will also be rotated by 90°.

In this particular embodiment, the optical component also comprises anactuation means (172, 174), arranged to change the overall orientationof the second layer 170. In this particular embodiment, the firstorientation of the layer 170 is defined by the alignment layers.However, the second orientation is provided by the actuating meansacting to apply an electric field across the second layer 170. Thedirectors within the second layer 170 will then align with the electricfield (provided it is large enough). In this particular example, theelectric field is arranged so as to be parallel to the optical axis 19.The electric field is provided by placing a voltage V_(s) across the twoelectrodes 150, 160. The voltage V_(s) is provided to the electrodes150, 160 by the voltage source 172 when the switch 174 is closed.

Spacers 164 act to define the width of the second layer 170, and toenclose the liquid crystal of the second layer. These spacers can beformed of any desired material, and can be formed of a transparentmaterial such as glass or foil.

FIGS. 6A and 6B illustrate this embodiment of the optical component,with the second layer 170 in respectively the first orientation and thesecond orientation. Detailed explanation of the effect of changing theorientation of the second layer is provided below with reference tothese figures.

FIGS. 2A-2F illustrate respective steps in forming a first portion of anoptical component in accordance with a preferred embodiment of thepresent invention. In this particular instance, the optical componentincludes a liquid crystal birefringent lens.

In the first step, shown in FIG. 2A, mould 100 is provided, the mouldhaving a shaped surface 102 which subsequently serves to define aportion of the shape of the resulting optical component. In thisparticular instance, the liquid crystal is ultimately photopolymerised,and consequently the mould is formed of a material transparent to theradiation used to polymerize the liquid crystal e.g. glass.

An alignment layer 110 is arranged on the curved surface 102, so as toinduce a predetermined orientation (indicated by the arrow direction110) in the liquid crystal subsequently placed upon the alignment layer.

In this particular example, the alignment layer is a layer of polyimide(PI). The polyimide may be applied using spincoating from a solution.The polyimide may then be aligned so as to induce a specific orientation(this orientation determining the resulting orientation of the liquidcrystal molecules). For instance, a known process is to rub thepolyimide layer with a non-fluff cloth repeatedly in a single directionso as to induce this orientation (110).

A substrate 150, which in this particular embodiment will form part ofthe optical component, comprises a layer of ITO upon a glass substrate.A bonding layer 120 is applied to a first surface 152 of the substrate150. The bonding layer is arranged to form a bond with the liquidcrystal. In this particular instance, the bonding layer is also analignment (or orientation) layer comprising polyimide. The bonding layercontains reactive groups arranged to form a chemical bond with theliquid crystal molecules, and in this instance has the same type ofreactive group as the liquid crystal molecules, such that whenphotopolymerising the liquid crystal molecules, chemical bonds with thebonding layer on the substrate are also created. This results in verygood adhesion between substrate and the liquid crystal layer. Thebonding layer may be deposited on the substrate using the same type ofprocess used to deposit and align the alignment layer on the mould 100.The bonding layer, which in this instance also functions as an alignmentlayer, is oriented in a predetermined orientation (arrow 120) dependingupon the desired properties of the resulting liquid crystal components.

The bonding layer is aligned so as to be parallel to the direction 110of the alignment layer on the mould. Preferably, the orientation of thebonding layer is parallel but in the opposite direction to theorientation of the alignment layer.

As illustrated in FIG. 2B, a compound 200 incorporating one or moreliquid crystals is then placed between the first surface 152 of thesubstrate 150 and the shaped surface 102 of the mould 100.

In this particular example, as illustrated in FIG. 2B, the compound 200comprises a mixture of two different liquid crystals. These twodifferent liquid crystals have been chosen so as to provide the desiredrefractive index properties once at least one of the liquid crystals hasbeen polymerized.

A droplet of the liquid crystals 200 is placed on the first surface 152of the substrate. The compound 200 has been degassed, so as to avoid theinclusion of air bubbles within the resulting optical component. It alsoavoids the formation of air bubbles from dissolved gases coming out ofthe solidifying liquid during polymerization, as the shrinkage duringpolymerization leads to a large pressure decrease inside thepolymerizing liquid.

The glass mould is then heated so that the liquid crystal is in theisotropic phase (typically to about 80° C.-120° C.), so as to facilitatethe subsequent flow of the liquid crystal into the desired shape.

The substrate and the mould are subsequently brought together, so as todefine the shape of the liquid crystal portion 201 of the finalresulting optical component (FIG. 2C). In order to ensure that theliquid crystal forms a homogenous layer between the mould and thesubstrate, a pressure may be applied to push the substrate towards themould (or vice versa).

The substrate/mould/liquid crystal may then be cooled, for instance downto room temperature for 30 minutes, so as to ensure that the liquidcrystal enters the nematic phase, coming from the isotropic phase.

When entering the nematic stage, multi domains may appear in the liquidcrystal mixture. Consequently, the mixture can be heated to above theclearing point to destroy the multidomain orientation (e.g. the mixturemay be heated for 3 minutes to 105° C.). Subsequently, the mixture maybe cooled to obtain a homogenous orientation 202 (FIG. 2D).

The homogenous liquid crystal mixture may then be photopolymerised usinglight 302 from an ultra violet radiation source 300 (FIG. 2E), forinstance by applying a UV-light intensity of 10 mW/cm² for 60 seconds.At the same time, chemical bonds will be formed between the liquidcrystal and the bonding layer.

Subsequently, the first element (or portion) of the optical component(150, 203) can be released from the mould 100 (FIG. 2F). This could, forinstance, be achieved by slightly bending the mould 100 over a corneredobject 400. Alternatively, it could be achieved by pressing a portion ofthe flat substrate in a flat support, so as to slightly bend the flatsubstrate. The liquid crystal/substrate element should separate easilyfrom the mould, as a conventional polyimide (without reactive groups) isused on the mould.

The mould can be reused to produce subsequent elements of components, byrepeating steps illustrated in FIGS. 2B-2F. Typically, the alignmentlayer will remain upon the mould 100, and hence does not need to bereapplied.

If desired, a further processing step can be performed to remove theliquid crystal 202 from the substrate 150. However, in most instances itis assumed that the substrate 150 will form part of the final opticalcomponent.

FIGS. 3A-3D illustrate successive steps in providing a secondbirefringent layer, so as to complete the optical component. Again, inthis particular embodiment, a liquid crystal is used to provide thesecond layer.

FIG. 3A shows a first alignment layer (a polyimide) being placed uponthe curved surface of the first birefringent layer 203. In thisparticular embodiment, the alignment layer is orientated so as to beperpendicular to the alignment of the directors within the firstbirefringent layer 203.

A second substrate 160 is provided substantially parallel to, but spacedapart from, the first portion of the component (150, 203). The substrate160 is used to form an electrode, and consequently is again formed ofglass and ITO. An alignment layer 162 is placed upon the surface of thesubstrate 160 adjacent to the curved surface 206 of the first layer 203.The alignment layer 162 is again formed of polyimide (PI). However, inthis instance, the polyimide layer 162 is arranged to be parallel to thedirectors in the first layer 203.

As shown in FIG. 3C, spacers 164 are arranged to space the twosubstrates 150, 160. The length of the spaces defines the distancebetween the substrate 150, 160, and hence the thickness of the secondlayer of birefringent material. The spacers are ultimately arranged,along with the substrates 150 and 160, and the first layer 203, so as toenclose the second birefringent layer 170. Consequently, spacers areglued all around the periphery of the substrates 150, 160, with only afill hole and an air hole being left.

Capillary cell filling is then used to fill the enclosed space via thefill hole. Subsequently, the fill hole and the air release hole areclosed (e.g. using a plug or glue), so as to form the resulting opticalcomponent 181. As indicated in FIG. 3D, the second layer 170 will orientso as to align with the alignment layer in the immediate vicinity.Consequently, as the alignment layers utilized were perpendicular toeach other, the second layer 170 exists in the twisted nematic state.

Using the above manufacturing method, an optical component has beenformed made of two birefringent materials between transparent conductivelayers. The second birefringent material is able to switch thepolarization of the incoming light beam actively by applying a voltageto the conductive layers. The other birefringent layer can be a passivelayer. The shaped interface between the two layers can be of any desiredshaped that provides an optical function, but in the preferredembodiment is a curved surface. The curvature of the surface is ofoptical quality to minimize aberrations.

In this particular preferred embodiment, so as to provide a multifocallens, the two materials have been selected such that the ordinary andextraordinary refractive indices of the active layer 170 arerespectively equal to the ordinary and extraordinary refractive indicesof the passive layer.

The voltage (V_(s)) that can be applied to the conductive layers issufficient so as to completely cancel the in-plane twist of the twistednematic state, and lead to the directors being aligned with the electricfield.

The result is an optical component, generally similar to thatillustrated in FIG. 1.

A suitable polyimide for use in the alignment layers is OPTMER AL-1051supplied by Japan Synthetic Rubber Co., whilst Durimide 7505 by ArchChemical can be used as an appropriate reactive polyimide withmethacrylate groups as the bonding layer.

The material used for the first (passive) layer preferably comprises areactive liquid crystal material. Preferably, the mesogenic group withthe liquid crystal is end or side capped by one or more polymerizablegroups. The material is able to exhibit a nematic phase within a certain(preferably relatively broad) temperature range. The polymerizable groupcan be a meth-acrylate, an acrylate, an oxirane, an oxitane, a vinylether, or any other reactive group.

As mentioned above, in the preferred embodiment a mixture of two liquidcrystals was utilized in the first layer 203 to obtain the desired n_(e)and n_(o). The two liquid crystals utilized were1,4-di(4-(3-acryloyloxypropyloxy)benzoyloxy)-2-methylbenzene (RM 257)and RM 82, both from Merck, Darmstadt, Germany. The photoinitiator usedto ensure the photo polymerisation of the liquid crystals in the firstlayer 203 was Irgacure 651, obtainable from Ciba Geigy, Basel,Switzerland.

The second layer (170) is preferably a nematic liquid crystal. Thesecond layer can be formed of E7 (a cyanobiphenyl mixture with a smallportion of cyanotriphenyl compound).

FIG. 4 shows a device 1 for scanning an optical record carrier 2,including an objective lens 18 according to an embodiment of the presentinvention. The record carrier comprises a transparent layer 3, on oneside of which an information layer 4 is arranged. The side of theinformation layer facing away from the transparent layer is protectedfrom environmental influences by a protection layer 5. The side of thetransparent layer facing the device is called the entrance face 6. Thetransparent layer 3 acts as a substrate for the record carrier byproviding mechanical support for the information layer.

Alternatively, the transparent layer may have the sole function ofprotecting the information layer, while the mechanical support isprovided by a layer on the other side of the information layer, forinstance by the protection layer 5 or by a further information layer anda transparent layer connected to the information layer 4. Informationmay be stored in the information layer 4 of the record carrier in theform of optically detectable marks arranged in substantially parallel,concentric or spiral tracks, not indicated in the Figure. The marks maybe in any optically readable form, e.g. in the form of pits, or areaswith a reflection coefficient or a direction of magnetization differentfrom their surroundings, or a combination of these forms.

The scanning device 1 comprises a radiation source 11 that can emit aradiation beam 12. The radiation source may be a semiconductor laser. Abeam splitter 13 reflects the diverging radiation beam 12 towards acollimator lens 14, which converts the diverging beam 12 into acollimated beam 15. The collimated beam 15 is incident on an objectivesystem 18.

The objective system may comprise one or more lenses and/or a grating.The objective system 18 has an optical axis 19. The objective system 18changes the beam 17 to a converging beam 20, incident on the entranceface 6 of the record carrier 2. The objective system has a sphericalaberration correction adapted for passage of the radiation beam throughthe thickness of the transparent layer 3. The converging beam 20 forms aspot 21 on the information layer 4. Radiation reflected by theinformation layer 4 forms a diverging beam 22, transformed into asubstantially collimated beam 23 by the objective system 18 andsubsequently into a converging beam 24 by the collimator lens 14. Thebeam splitter 13 separates the forward and reflected beams bytransmitting at least part of the converging beam 24 towards a detectionsystem 25. The detection system captures the radiation and converts itinto electrical output signals 26. A signal processor 27 converts theseoutput signals to various other signals.

One of the signals is an information signal 28, the value of whichrepresents information read from the information layer 4. Theinformation signal is processed by an information processing unit forerror correction 29. Other signals from the signal processor 27 are thefocus error signal and radial error signal 30. The focus error signalrepresents the axial difference in height between the spot 21 and theinformation layer 4. The radial error signal represents the distance inthe plane of the information layer 4 between the spot 21 and the centerof a track in the information layer to be followed by the spot.

The focus error signal and the radial error signal are fed into a servocircuit 31, which converts these signals to servo control signals 32 forcontrolling a focus actuator and a radial actuator respectively. Theactuators are not shown in the Figure. The focus actuator controls theposition of the objective system 18 in the focus direction 33, therebycontrolling the actual position of the spot 21 such that it coincidessubstantially with the plane of the information layer 4. The radialactuator controls the position of the objective lens 18 in a radialdirection 34, thereby controlling the radial position of the spot 21such that it coincides substantially with the central line of track tobe followed in the information layer 4. The tracks in the Figure run ina direction perpendicular to the plane of the Figure.

The device of FIG. 4 in this particular embodiment is adapted to scanalso a second type of record carrier having a thicker transparent layerthan the record carrier 2. The device may use the radiation beam 12 or aradiation beam having a different wavelength for scanning the recordcarrier of the second type. The NA of this radiation beam may be adaptedto the type of record carrier. The spherical aberration compensation ofthe objective system must be adapted accordingly.

FIG. 5 illustrates an optical component 181 in accordance with apreferred embodiment of the present invention in use within the scanningdevice 1. FIGS. 6A and 6B illustrate the two extreme orientations of thesecond layer of the liquid crystal (although the liquid crystal is infact continuously variable in a controlled manner between these twoextremes by varying the voltage applied between zero and V_(s)).

As shown in FIG. 5, the optical component 181 can be placed within theobjective system 18 of a scanning device. By appropriate control of thepolarisation of the parallel beam 15, as well as by controlling theorientation of the second layer 170 within the device, the objectivesystem 18 can be used to scan at different layers 4 a, 4 b, 4 c, 4 d . .. within a multi-layer disc 2′.

The objective system 18 comprises the optical component 181 and afocusing lens 182. The focusing lens 182 is arranged to focus the beamfrom the optical component 181 (which may be parallel, diverging orconverging) to a spot on the correct information layer. The opticalcomponent 181 acts to alter the parallel polarised beam 15 to thecorrect diverging, converging, or parallel state dependent upon thedesired information layer 4 a, 4 b, 4 c, 4 d, . . . to be scanned.Optionally, the objective system 18 may also comprise a polariser forpolarisation selection of the beam from the optical element 181 (in someinstances the beam from the optical element may be split into twodirections, depending upon the state of the optical component).

FIG. 6A illustrates the lens 181 with the second layer 170 in thetwisted nematic state (i.e. no voltage is applied to the electrodes 150,160). In FIG. 6B, a voltage V_(s) has been applied so as to induce anelectric field between the electrodes 150, 160. The electric field ishigh enough for complete cancellation of the in-plane twist of thebirefringent layer 170.

It will be appreciated that the optical properties of the lens 181 willvary depending upon the orientation of the layer 170. Further, theoptical properties will of course vary depending upon the refractiveindices between the layers. In this particular embodiment, therefractive indices of the birefringent layer 170 have been chosen tomatch the respective refractive indices (n_(o), n_(e)) of the passivelayer 203.

The optical function provided by the lens 181 will vary in dependenceupon the polarisation of the incoming light (e.g. whether thepolarisation state of the incoming light is parallel to the direction ofthe directors within the passive layer 203, or perpendicular to thedirection of the directors within the passive layer 203), and upon thedirection of the incident light i.e. whether the light is incident firstupon the passive layer 203 (as indicated by the arrow A), or whether thelight is incident first upon the active layer 170 (as indicated by thearrow B). Using the notation that the “off state” corresponds to novoltage being applied (as shown in FIG. 6A), and the “on state”corresponds to a voltage being applied sufficient to completely cancelthe in-plane twist (i.e. FIG. 6B), then the following conditions can beseen to exist:

(1) Light Entering the Lens Via the Passive Layer

(Direction A)

-   (i) Off state and incoming polarization state of the light is    parallel to directors of the passive layer on entrance: a shift from    n_(e) to n_(o) results at the interface; the curved surface hence    acts as a positive lens. Further in the active layer the    polarization is rotated 90°.-   (ii) Off state and incoming polarization state of the light is    perpendicular to directors of the passive layer on entrance: a shift    from n_(o) to n_(e) results at the interface; the curved surface    hence acts as a negative lens. Further in the active layer the    polarization is rotated 90°.-   (iii) On state and incoming polarization state of the light is    parallel to the directors of the passive layer on entrance: a shift    from n_(e) to n_(o) results at the interface; the curved surface    hence acts as a positive lens. No further change of the    polarization.-   (iv) On state and incoming polarization state of the light is    perpendicular to the directors of the passive layer on entrance: No    shift occurs (n_(o) to n_(o)) at the interface; the curved surface    hence acts as a neutral lens. No further change of the polarization.-   (v) In between the on and off state, refractive indices can be    selected between n_(e) and n_(o), resulting in a lens that is    multi-focal from positive to neutral without the use of an extra    selection polarizer. The polarization only changes in the second    layer (the active layer). For fluorescent recording this change in    polarization is not important.    (2) Light Entering the Lens Via the Active Layer (Direction B):-   (i) Off state and incoming polarization state of the light is    parallel to the directors of the passive layer on entrance:    polarization is rotated 90° and light will enter the passive layer    with a polarization state that is perpendicular to the directors of    the passive layer. This means a shift from n_(e) to n_(o) on the    interface between the two layers. In combination with the curvature    on the interface between active and passive layer this results in a    negative lens.-   (ii) Off state and incoming polarization state of the light is    perpendicular to the directors of the passive layer on entrance:    polarization is rotated 90° and so light will enter the passive    layer with a polarization state that is parallel to the directors of    the passive layer. This means a shift from n_(o) to n_(e) at the    interface between the two layers. In combination with the curvature    on the interface between active and passive layer this results in a    positive lens.-   (iii) On state and incoming polarization state of the light is    parallel to the directors of the passive layer on entrance: no    rotation of the polarization. A shift from n_(o) to n_(e) results at    the interface; the curved surface hence acts as a positive lens.-   (iv) On state and incoming polarization state of the light is    perpendicular to the directors of the passive layer on entrance: no    rotation of the polarization. No shift occurs (n_(o) to n_(e)) at    the interface; the curved surface hence acts as a neutral lens.-   (v) In between the on and the off state, partial polarization shifts    will occur and refractive indices can be selected between n_(e) and    n_(o). Because of the partial polarization shift the laser beam will    enter the passive layer with a polarization state that is not fully    perpendicular nor fully parallel (the light ray is dissolved in two    directions when the laser beam enters the passive layer). For this    reason a polarization selection should be used after the element, to    enable multi-focal behavior without having the result of both    polarizations at a time. This polarization selection can be done    with a separate polarizer.

It will be appreciated that the above embodiments are described by wayof example only, and that various alternatives will be apparent to theskilled person.

Whilst specific examples of materials suitable for forming the opticalcomponent have been described, and particular manufacturing steps, theseare again provided by way of example only.

The mould used in the manufacturing process may be formed of anymaterial, including rigid materials such as glass.

Further, the shaped surface of the mould may be dimensioned so as toallow for any change in shape or volume of the liquid crystal materialduring the method. For instance, typically liquid crystal monomersshrink slightly upon polymerisation, due to double bonds within theliquid crystal being reformed as single bonds. By appropriately makingthe optical component shaped defined by the substrate and the mouldslightly oversize, an appropriately sized and shaped optical componentcan be produced.

Whilst the substrates have been seen in this particular example ascomprising a single sheet of glass, with two flat, substantiallyparallel sides, it will be appreciated that the substrates can in factbe any desired shape.

An extra adhesion layer may be applied to the mould and/or substrate(prior to deposition of the bonding layer onto the substrate and theorientation layer to the mould), so as to make sure that the appliedlayers are well attached to the mould and the substrate. For instance,organosilanes may be used to provide this adhesion layer. For thesubstrate an organosilane comprising a methacrylate group may be usedand for the mould an organosilane comprising an amine end group may beused.

It will be appreciated that the above described optical components arealso described by way of example only. An optical component (or indeed,an optical element formed according to the present invention i.e. aportion of an optical component) could be formed with differentproperties to that described above, or of different birefringentmaterials.

For instance, in the above embodiments, it is assumed that therefractive indices of the second layer 170 of the component 180 areequal to the corresponding refractive indices of the first layer 203.However, it will be appreciated that in fact any values of ordinary andextraordinary refractive indices could be used for each layer. Forinstance, an optical component could be formed with the ordinaryrefractive index of one layer equal to the extraordinary refractiveindex of the other layer.

Equally, whilst in the above embodiments the optical component has beendescribed as having a curved interface between the two materials, itwill be appreciated that the interface could in fact be of any shapethat provides an optical function. For instance, the interface could bea step structure or a grating structure. In such instances, the opticalfunctions of the components can still be changed by the incidentpolarisation states and/or the orientation of the second layer.

In the preferred embodiment, it is assumed that the outer surfaces ofthe optical element (i.e. the surfaces upon which the light enters andexits the element) are two flat, parallel surfaces. However, thesesurfaces could in fact be any desired shape, including concave orconvex.

Equally, the second layer has been generally described as beingswitchable between two particular orientations, but it will beappreciated that the second layer can in fact be switchable between anynumber of orientations. Further, the first layer could be of anypredetermined orientation, and in fact if desired the first layer couldalso be an active layer i.e. it too could have a changeable orientation.

Preferably, the active layer(s) is continuously controllably variablebetween the two predetermined orientations. For instance, in theparticular embodiments illustrated, the orientations of the second layeris continuously variable between the two states shown in FIGS. 6A and 6Bby providing the appropriate voltage to the two electrodes.

Further, although in the preferred embodiment one of the orientationstates of the second layer is seen to be defined by alignment layerssubstantially perpendicular to the optical axis, it will be appreciatedthat these alignment layers could in fact be of any predeterminedorientation. For instance, the alignment layers could be parallel to theoptical axis e.g. by placing the alignment layers upon the internalsurfaces of the spacers 164. If desired, no alignment layers could beused to define an orientation of the second layer. Instead, electrodescould be used to define both orientations (for instance by placinganother set of electrodes within the spaces 164).

In all of the above embodiments, an optical component is providedcomprising at least two adjacent birefringent materials separated by ashaped interface. The orientation of at least one of the birefringentmaterials may be changed, so as to result in a change of function (e.g.lens strength, or type) of the shaped interface. Consequently, thefunction of the interface can be changed by changing both thepolarisation of the incident light, and by changing the orientation ofthe birefringent layer. The optical component can thus be used in arange of novel and interesting ways.

1. An optical component comprises a first birefringent layer connectedto a second birefringent layer by a shaped interface, an optical axispassing through the first and the second layer, at least the secondbirefringent layer having molecules movable between a first orientationand a second orientation relative to the optical axis, the refractiveindex of the second birefringent layer being dependent upon theorientation of the molecules.
 2. An optical component as claimed inclaim 1, wherein said interface is a curved interface.
 3. An opticalcomponent as claimed in claim 1, wherein the first birefringent layerhas an ordinary axis substantially perpendicular to the optical axis andan extraordinary axis substantially perpendicular to the optical axis.4. An optical component as claimed in claim 1, wherein at least one ofthe first layer and the second layer comprises a liquid crystal.
 5. Anoptical component as claimed in claim 1, wherein the second layercomprises a liquid crystal in the nematic phase.
 6. An optical componentas claimed in claim 1, wherein in the first orientation, the angle ofthe molecules relative to the optical axis changes as a function ofdistance along the optical axis.
 7. An optical component as claimed inclaim 1, wherein the second layer comprises a liquid crystal, with thefirst orientation corresponding to the liquid crystal being in thetwisted nematic phase.
 8. An optical component as claimed in claim 1,wherein the second orientation corresponds to the second layer havingthe extraordinary axis parallel to the optical axis.
 9. An opticalcomponent as claimed in claim 1, further comprising actuation means,arranged to change the orientation of the molecules.
 10. An opticalcomponent as claimed in claim 9, wherein said actuation means comprisesat least two electrodes arranged to apply an electric field to thesecond layer.
 11. An optical scanning device for scanning an informationlayer of an optical record carrier, the device comprising a radiationsource for generating a radiation beam and an objective system forconverging the radiation beam on the information layer, wherein thedevice comprises an optical component, the optical component comprisinga first birefringent layer connected to a second birefringent layer by ashaped interface, an optical axis passing through the first and thesecond layer, at least the second birefringent layer having moleculesmovable between a first orientation and a second orientation relative tothe optical axis, the refractive index of the second birefringent layerbeing dependent upon the orientation of the modules.
 12. A device asclaimed in claim 11, wherein the optical component forms a controllablelens within the objective system.
 13. A method of manufacturing anoptical component comprising a first birefringent layer and a secondbirefringent layer, the method comprising: providing a firstbirefringent layer with a shaped surface; providing a secondbirefringent layer adjacent to the shaped surface of the firstbirefringent layer; wherein the molecules of the second birefringentlayer are arranged to be movable between a first orientation and asecond orientation relative to an optical axis passing through the firstbirefringent layer and the second birefringent layer.
 14. A method asclaimed in claim 13, wherein the second birefringent layer is providedby capillary cell filling.
 15. A method of manufacturing an opticalscanning device for scanning an information layer of an optical recordcarrier, the method comprising: providing a radiation source forgenerating a radiation beam; providing an objective system forconverging the radiation beam on the information layer; and providing anoptical component, the optical component comprising a first birefringentlayer connected to a second birefringent layer by a shaped interface, anoptical axis passing through the first and the second layer, at leastthe second birefringent layer having molecules movable between a firstorientation and a second orientation relative to the optical axis, therefractive index of the second birefringent layer being dependent uponthe orientation of the modules.